Methods for producing single crystal silicon include the CZ method.
FIG. 1 shows an example structure of a single crystal pulling apparatus 1 using the CZ method.
A quartz crucible 3 of which outside is covered with a graphite crucible 11 is disposed in a single crystal pulling vessel 2 or a CZ furnace 2. Polycrystalline silicon (Si) is heated to melt in the quartz crucible 3. After melting is stabilized, a pulling mechanism 4 operates to pull a single crystal silicon (single crystal silicon ingot) 6 from melt 5. Specifically, a pulling shaft 4a is lowered, and a seed crystal 14 held by a seed chuck 4c at a leading end of the pulling shaft 4a is immersed in the melt 5. After the seed crystal 14 is adapted to the melt 5, the pulling shaft 4a is lifted. The single crystal silicon 6 grows as the seed crystal 14 held by the seed chuck 4c is pulled up. The quartz crucible 3 is rotated by a rotating shaft 10 while pulling up. The pulling shaft 4a of the pulling mechanism 4 rotates in an opposite direction or the same direction to the rotating shaft 10.
Various vaporized materials are produced within the vessel 2 during the single crystal pulling process (1 batch). Then, argon (Ar) gas 7 is supplied to the single crystal pulling vessel 2 and discharged together with the vaporized materials out of the vessel 2 to remove the vaporized materials from and clean the vessel 2. A supply flow rate of the argon gas 7 is determined for each process of the single batch.
A heat shielding plate 8 (gas rectifying column) is disposed above the quartz crucible 3 and around the single crystal silicon 6 to rectify the gas 7 in the single crystal pulling vessel 2, to guide to a surface 5a of the melt 5 and to shield the single crystal silicon 6 from a heat source. A distance (gap) G between the bottom end of the heat shielding plate 8 and the melt surface 5a is determined appropriately.
One of problems unavoidable when the single crystal silicon 6 is grown by the CZ method is a “dislocation” generated in the seed crystal at the time the seed crystal is immersed in the melt. The dislocation is induced because of a thermal stress caused in the seed crystal 14 when the seed crystal 14 is immersed in the melt 5. If the dislocation is continuously propagated through a necking portion formed at a lower portion of the seed crystal 14 and taken into the single crystal silicon 6 which is produced with the necking portion increased, this single crystal silicon 6 cannot be used for a semiconductor device. Therefore, it is necessary to eliminate the dislocation.
Accordingly, it is general that a necking process for gradually contracting the necking portion to a diameter of about 3 to 5 mm is conducted in the first step of the pulling process in order to remove the dislocation, which is introduced into the seed crystal 14 at the time the seed crystal is immersed in the melt, out of the seed crystal 14 after the seed crystal 14 is immersed in the melt 5.
In recent years, however, the production of a large-diameter silicon wafer having a diameter of 300 mm or more is being demanded, and a large-diameter and heavy single crystal silicon ingot is required to be pulled up without involving any problem. When a diameter of the necking portion is contracted to about 3 to 5 mm by the necking process, the dislocation is eliminated, but the diameter is excessively small, so that the large-diameter and heavy single crystal silicon ingot may not be produced without involving a problem that the crystal drops, or the like.
Here, an auxiliary holding device may be disposed to hold and pull the single crystal silicon 6 to prevent the single crystal silicon ingot from dropping due to breakage of the necking portion.
But, the addition of such an auxiliary holding device to the existing single crystal pulling apparatus 1 involves the increase of the number of parts and increases the apparatus cost. It is also presumed that technical difficulties are involved because a technique of securely holding the single crystal silicon 6 has not been established. Besides, when the mechanical holding device is disposed, a contaminant such as metal powder may be introduced into the CZ furnace 2, and the interior of the CZ furnace 2 might not be assured to have a clean environment.
Therefore, it is not desirable to dispose an auxiliary holding device to pull up the large-diameter and heavy single crystal silicon ingot.
Japanese Patent Application Laid-Open No. 11-189488 discloses the following technology that a large-diameter and heavy single crystal silicon ingot can be pulled up without reducing the necking portion.
a) The distance (gap) G between the bottom end of the heat shielding plate 8 and the melt surface 5a is increased to add radiant heat from the heater for heating the melt 5 in the quartz crucible 3 in a large amount to the seed crystal 14 so to raise the temperature of the seed crystal 14, thereby reducing a temperature difference between the seed crystal 14 and the melt 5 and reducing dislocations introduced into seed crystal 14 by a thermal stress.
b) A slit is formed in the graphite crucible 11 to add radiant heat from the heater for heating the melt 5 in the quartz crucible 3 in a large amount to the seed crystal 14 so to raise the temperature of the seed crystal 14, thereby reducing the temperature difference between the seed crystal 14 and the melt 5 and reducing dislocations introduced into the seed crystal 14 by a thermal stress.
c) An auxiliary heating device which is vertically movable by a moving mechanism is disposed, the auxiliary heating device is disposed on the side of the seed crystal 14 in the necking process to heat the seed crystal 14 so to raise its temperature to 1380° C. to 1480° C., thereby reducing the temperature difference between the seed crystal 14 and the melt 5 and reducing dislocations introduced into the seed crystal 14 by a thermal stress. As a test result, the necking portion was not broken when the seed crystal 14 had a diameter of 8 mm or 14 mm.
It is seen from the above a) and b) that it is advisable to raise the temperature of the seed crystal 14, but there is no suggestion about a quantitative value to which the temperature of the seed crystal 14 must be raised.
According to the above-described c), the seed crystal 14 is raised to the temperatures of 1380° C. to 1480° C., but if the seed crystal 14 is raised to such a high temperature, the seed crystal 14 might melt before it is immersed in the melt 5, and the diameter of the seed crystal 14 before it is immersed in the melt might be contracted. There is also a possibility that the leading end of the melted seed crystal drops to the melt 5 to change a dopant concentration in the melt 5, and a target crystal resistivity might be deviated. Besides, when the temperature is raised to such a high level, the quartz crucible may be deformed, and the crystal might be deformed while it is being pulled, and a possibility that dislocations are introduced into the crystal becomes high. In other words, 1380° C. to 1480° C. to which the temperature of the seed crystal 14 is raised are values of over-specification in eliminating the dislocations.
Besides, it is seen from the above-described c) that the necking portion is not broken when the seed crystal 14 having the diameter of 8 mm or 14 mm is raised to the temperatures of 1380° C. to 1480° C., but a critical relationship between the diameter of the seed crystal 14 and the temperature of the seed crystal 14 that the necking portion is not broken is not shown clearly.
According to the above-described a), increase of the gap distance G between the bottom end of the heat shielding plate 8 and the melt surface 5a is effective, however, this distance G is a parameter limiting an amount of oxygen vaporizing from the melt surface 5a, and the oxygen concentration taken into the single crystal silicon 6 is influenced depending on the size of the gap G. Therefore, the dislocations introduced into the single crystal silicon 6 when the gap G is increased in size can be eliminated, but the oxygen concentration in the single crystal silicon 6 is also influenced, possibly affecting significantly on the characteristics of an element or a device. When the size of the gap G is changed, the temperature of the melt 5 is also affected. Therefore, it is not desirable to adopt a technique of adjusting the gap G.
According to the above-described b), when the slit is formed in the graphite crucible 11, the temperature of the melt 5 might be raised. Therefore, it is not desirable to adopt the technique of forming the slit in the graphite crucible 11.
According to the above-described c), the addition of the moving mechanism and the auxiliary heating device to the existing single crystal pulling apparatus 1 increases the number of parts and increases the apparatus cost. Therefore, it is not desirable to adopt the technique of disposing such new moving mechanism and auxiliary heating device to heat merely the seed crystal 14.